“African Rambo” are the two words that best summarize Marc Forster’s “Machine Gun Preacher,” but they would actually make a better title. They’re spoken by a clueless hillbilly archetype to describe the emotionally charged Sam Childers (Gerard Butler), the real-life junkie-turned-pastor whose newfound faith led him to found an orphanage in Sudan. Forster presents Childers as a bible-touting superhero, which might work if the movie didn’t possess more serious aims it fails to achieve.

There’s no doubting the extraordinary, even sensationalistic appeal of Childers, a former bike-gang member, criminal and addict whose escape began with his born-again status in the mid-1980s and led to his missionary trips to Sudan in the late 1990’s. Childers recounts his saga in the memoir “Another Man’s War: The True Story of One Man’s Battle to Save Children in Sudan,” and it’s easy to see why the middle-aged warrior’s journey lends itself to the Hollywood treatment. However, while this is compelling material on the page, in movie terms it translates into a seriously conflicted work ruined by spectacular overstatement.

Childers went beyond his biblical calling by leading armed resistance against Sudanese militants, evolving into a badass Christian on a mission. An article on Global Tryst, a site devoted to world-peace focus projects, describes him in terms that evoke B-movie icons: “With a physique like Jean Claude Van Damme… Childers has hunted alligators in the U.S. and smacked down miscreants in Africa. This titan [could] easily pass for Hulk Hogan’s younger brother.” Ironically, that iconography sets the bar too high for Butler, but “Machine Gun Preacher” suffers from deeper problems than its casting.

Forster’s ability to navigate racy waters stretches back to “Monster’s Ball,” which explored racial and class tensions with more caution. But these days, his forthcoming zombie epic, “World War Z” is more his speed. The action scenes in “Machine Gun Preacher” work fine on their own, but they cheapen a work that attempts to command great importance.

Childers’ experience requires dual narratives. The first one focuses on his drug-addled beginnings and the toll it takes on his devout Christian wife (Michelle Monohan). These scenes at least maintain a consistency for their sheer grittiness thanks to a major assist from Michael Shannon, who provides a characteristically maniacal turn as Childers’ partner in crime. Then Childers heads to Sudan and discovers the oppressive sins of a local militia known as the Lord’s Resistance Army. Here, Childers’ increasing devotion to the cause and eventual decision to fight back favors flashy editing strategies over plausibility.

Initially, the blunt storytelling is enlivened by Butler’s ferocious performance — if it’s taken at face value as a one-note figure with the credibility of “The Punisher,” and if you can accept “Machine Gun Preacher” as being “Higher Ground” with guns. But Forster has created a B-movie with delusions of grandeur. Like the white doctor in Susanne Biers’ Oscar-winning tearjerker “In a Better World,” Childers’ dominating presence turns the victims into a representational crowd rather than bonafide human beings (there’s only one non-white character with a developed personality, even though dozens of others flood the frame).

The fallacy plaguing many of these white-guilt narratives is the assumption that a true-life basis validates everything onscreen. “Machine Gun Preacher” allows Childers’ one-man-army approach to speak for itself, which distances it from the story’s non-fiction roots. The script suffers from a constant identity crisis: No matter how real the twists, it can’t avoid the mutually troublesome pratfalls of didacticism and incredulity. Guns a-blazing, “Machine Gun Preacher” repeatedly shoots itself in the foot.

criticWIRE grade: C

HOW WILL IT PLAY? With a flashy trailer and Butler’s star power, “Machine Gun Preacher” stands to do decent business this weekend at the box office, but mixed word-of-mouth will probably hold back its continuing commercial prospects.